of cases. Less frequently, it results from some malignant tumors, such as osteosarcoma, chondrosarcoma, and hemangioendothelioma. Pathology Histologically, they are classified in two variants. The classic (or standard) form (95%) has blood filled clefts among bony trabeculae. Osteoid tissue is found in stromal matrix. The solid form (5%) shows fibroblastic proliferation, osteoid production and degenerated calcifying fibromyxoid elements. According to Buraczewski and Dabska, the development of the aneurysmal bone cyst follows three stages. They can also be associated with a TRE17/USP6 translocation. Aneurysmal bone cysts may be intraosseous, staying inside of the bone marrow. Or they may be extraosseous, developing on the surface of the bone, and extending into the marrow. A radiograph will reveal a soap bubble appearance. Diagnosis X-ray and CT scan show lytic expansion lesions with clear borders. Expansion of cortex gives the lesion a balloon-like appearance. Larger lesions may appear septated. MRI reveals fluid levels. Bone scan shows outer radiotracer uptake, with a central dark area. Differential diagnosis Following conditions are excluded before diagnosis can be confirmed: Unicameral bone cyst Giant cell tumor Telangiectatic osteosarcoma Secondary aneurysmal bone cyst Treatment Curettage is performed on some people, and is sufficient for inactive lesions. The recurrence rate with curettage is significant in active lesions, and marginal resection has been advised. Liquid nitrogen, phenol, methyl methacrylate are considered for use to kill cells at margins of resected cyst. Prognosis 20–70% recur after curettage. Epidemiology It is rare. The incidence is around 0.15 cases per one million per year. 80% occur in people age less than 20 years. It is second most common tumor of spine and most common benign tumor of pelvis in pediatric population. Males are affected as equally as females. Additional images See also Simple bone cyst (SBC) Giant cell tumor of bone Traumatic bone cyst References External links Osteopathies Vascular neoplasia